The anisotropic magnetoresistance effect in 3d transition metals and alloys is reviewed. This effect, found in ferromagnets, depends on the orientation of the magnetization with respect to the electric current direction in the material. At room temperature, the anisotropic resistance in alloys of Ni-Fe and Ni-Co can be greater than 5%. The theoretical basis takes into account spin orbit coupling and d band splitting. Other properties such as permeability, magnetostriction, and Hall voltage have no simple relationship to magnetoresistance. Anisotropic magnetoresistance has an important use as a magnetic field detector for digital recording and magnetic bubbles. Such detectors because of their small size are fabricated using thin film technology. Film studies show that thickness, grain size, and deposition parameters play a significant role in determining the percentage change in magnetoresistance. In general, the change is smaller in films than bulk materials. Several tables and graphs that list bulk and film data are presented.

Anisotropic magnetoresistance in ferromagnetic 3d alloys

Despite common misconceptions, domain walls are too thick to ’’scatter’’ electrons appreciably. However, electrons crossing a wall apply a torque to it, which tends to cant the wall spins. This could be used to measure the conduction electron spin polarization. Most of the low‐field resistive anomalies observed in pure Fe, Ni and Co at low temperature are caused by the Lorentz force associated with the internal field B=Ms present inside each domain. The existence of low‐resistivity paths extending over many domains accounts for still unexplained magnetoresistance data in iron whiskers. In uniaxial materials, a d.c. eddy‐current loop caused by the Hall effect runs around each wall. The field Hz generated by these loops tends to ’’drag’’ the whole domain structure in the direction of the carrier drift velocity. Also, the Joule dissipation of the eddy currents manifests itself as an excess Ohmic resistance. As predicted, this excess resistance decreases as the square of the field, in amorphous Gd25Co75 films...

Low‐field magnetoresistance and domain drag in ferromagnets

We built and measured the dynamical current versus time behavior of nanoscale niobium oxide crosspoint devices which exhibited threshold switching (current-controlled negative differential resistance) The switching speeds of 110 × 110 nm(2) devices were found to be Δt(ON) = 700 ps and Δt(OFF) = 2:3 ns while the switching energies were of the order of 100 fJ We derived a new dynamical model based on the Joule heating rate of a thermally driven insulator-to-metal phase transition that accurately reproduced the experimental results, and employed the model to estimate the switching time and energy scaling behavior of such devices down to the 10 nm scale These results indicate that threshold switches could be of practical interest in hybrid CMOS nanoelectronic circuits

https://iopscience.iop.org/article/10.1088/0957-4484/23/21/215202/pdf

Sub-100 fJ and sub-nanosecond thermally driven threshold switching in niobium oxide crosspoint nanodevices.

An Nb‐Sn filament mounted on a flexible glass beam can be broken to form an electron tunneling junction between the fracture elements. Breaking the filament in liquid helium prevents oxidation of the freshly exposed fracture surfaces. A sharp superconducting energy gap in the I‐V characteristics measured at 4 K indicates the formation of a high‐quality tunneling barrier between the fracture elements. The resistance of the junction can be continuously adjusted by varying the surface bending strain of the beam. An estimated 0.1 nm change in the barrier thickness produces about an order of magnitude change in the resistance over the range from 105 to 108 Ω. The exponential character of this dependence shows that the tunnel junction is freely adjustable without intimate contact of the junction elements. ‘‘Break’’ junctions made in this way offer a new class of tunneling experiments on freshly exposed surfaces of a fractured sample without the oxide barrier previously required for junction stability. Such expe...

Electron tunneling experiments using Nb‐Sn ‘‘break’’ junctions

A number of important commercial applications would benefit from the introduction of easily manufactured devices that exhibit current-controlled, or “S-type,” negative differential resistance (NDR). A leading example is emerging non-volatile memory based on crossbar array architectures. Due to the inherently linear current vs. voltage characteristics of candidate non-volatile memristor memory elements, individual memory cells in these crossbar arrays can be addressed only if a highly non-linear circuit element, termed a “selector,” is incorporated in the cell. Selectors based on a layer of niobium oxide sandwiched between two electrodes have been investigated by a number of groups because the NDR they exhibit provides a promisingly large non-linearity. We have developed a highly accurate compact dynamical model for their electrical conduction that shows that the NDR in these devices results from a thermal feedback mechanism. A series of electrothermal measurements and numerical simulations corroborate this model. These results reveal that the leakage currents can be minimized by thermally isolating the selector or by incorporating materials with larger activation energies for electron motion.

/pdf/an-accurate-locally-active-memristor-model-for-s-type-4ke6mqj0kt.pdf

An accurate locally active memristor model for S-type negative differential resistance in NbOx

Conduction Phenomena in Thin Layers of Iron Oxide

Results on tunnel current flow in junctions using ferromagnetic metals as electrodes or as added impurities at the barrier electrode interface are reported in this paper. Oxides of the ferromagnetic metal and aluminum oxide have been used to form the insulating barrier in combination with a second electrode made from aluminum, silver, or iron. Complete data are reported for various types of junctions using iron and iron oxide and covering a zero-bias resistance range at 4.2\ifmmode^\circ\else\textdegree\fi{}K from ${10}^{\ensuremath{-}1}$ \ensuremath{\Omega} to above ${10}^{8}$ \ensuremath{\Omega}. Preliminary results on junctions using cobalt and nickel are also reported. The most general feature of interest is a very strong non-Ohmic behavior at low bias voltage and a very rapid temperature dependence of tunnel conductance, particularly in the helium temperature range. Data obtained on Al-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-Al junctions doped with iron impurities at the ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-Al interface show a strong zero-bias anomaly corresponding to a resistance maximum. This behavior appears to be generally explained by an anomalous scattering mechanism near zero bias and has been compared to the behavior of identical undoped Al-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-Al junctions. The experiments on junctions using iron oxide barriers also show strong zero-bias anomalies, but in addition show a more complex temperature dependence and an unusual breakdown effect at high voltages. These latter features are possibly associated with special properties of magnetic iron oxide ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$. The data on junctions with iron oxide barriers have also been compared to the behavior predicted by standard tunnel theory, and areas of qualitative agreement can be found if a relatively low barrier height is assumed. The appropriate barrier height has been estimated as 0.025 eV from Fowler-Nordheim plots of the data. The preliminary data on junctions made with $\mathrm{Co}{\mathrm{O}}_{x}$ and $\mathrm{Ni}{\mathrm{O}}_{x}$ barriers also show some similar effects, but they are generally of smaller magnitude. The experimental data will be presented in detail along with a summary of the relevant theories.

Experiments with Tunnel Junctions Using Ferromagnetic Metals

Galvanomagnetic effects have been measured and compared in single‐crystal iron films 0.1 to 5 μ in thickness and in iron whiskers 10 to 500 μ in thickness. Measurements have been made in a temperature range 1° to 300°K and in magnetic fields to 80 kOe, although in this paper we discuss primarily the low‐field negative magnetoresistance observed at helium temperatures. The negative magnetoresistance develops when the mean free path of the electron becomes long. In the case of whiskers with RRR≈1000 the effect can represent a 90% reduction in resistance at 4.2°K, while with films having RRR≈10–50 the effect is reduced to 1%. The negative magnetoresistance in 〈100〉 axial whiskers is very sensitive to measuring current, while the negative magnetoresistance in 〈111〉 axial whiskers is not as sensitive, although both cases show substantial deviations from Ohm's law. Analysis of the longitudinal and transverse data on the films indicates that the reverse galvanomagnetic effect is making a substantial contribution...

Galvanomagnetic Properties of Single‐Crystal Iron Films and Whiskers

Zero bias anomalies in AlAl2O3Al tunnel junctions doped with Fe

Magnetoresistance measurements on single-crystal nickel and iron films have been made at temperatures of 4.2 to 295 K in fields of up to 50 kOe. The films were prepared by vacuum deposition onto heated rock salt and mica substrates producing films with (100) and (111) planes respectively, parallel to the film surface. The films varied in thickness from 500 A to 5 μ and had resistivity ratios, ρ295/ρ4.2, varying from 9 to 55. Transverse magnetoresistance curves taken at 295 K show crystalline anisotropies similar to those observed in bulk samples. At lower temperatures, the normal positive magnetoresistance is obtained, but anisotropies associated with the Fermi surface topology are not as marked as with bulk samples, due to the lower electron mean free paths of the electrons in film specimens. Both longitudinal and transverse magnetoresistance hysteresis curves were obtained for (100) nickel and iron films. An increased coercive force is obtained as the temperature is reduced for both nickel and iron films.

J. E. Christopher

Papers

Conduction Phenomena in Thin Layers of Iron Oxide

Experiments with Tunnel Junctions Using Ferromagnetic Metals

Galvanomagnetic Properties of Single‐Crystal Iron Films and Whiskers

Zero bias anomalies in AlAl2O3Al tunnel junctions doped with Fe

Magnetoresistance of Nickel and Iron Single-Crystal Films